Digital systems for generating printed media have become popular in the field of graphic arts printing. Typically, the systems use a digital database from which print forms are generated and deposited either onto a plate which is subsequently mounted in a press or on the print cylinder of a press. In both cases, the print information may be recorded as binary signals which collectively represent the "signature image". These plates or cylinders are always separated in terms of the principle color components of the original image, e.g., cyan, magenta, yellow and black. The color components can be produced sequentially or simultaneously with parallel recording heads. The recording heads that are used in prior art apparatus feature 1) multiple laser beams which sweep transversely across the plate or cylinder at high speed line by line, 2) multiple laser diodes which traverse the recording medium while writing multiple lines in helical fashion, or 3) arrays of light emitting diodes (LEDs) to record serially a helically pattern which represents a mono-color page.
In each case of the prior art, the recording medium is light sensitive; this requires that all prior art apparati have a light-tight recording and printing chamber to avoid accidental exposure of the recording medium. The first approach uses a waterless method to pick up offset ink which is subsequently transferred to the printing substrate. The second approach uses a special liquid electrostatic toner comprising charged particles which are deposited electrostatically on the print member and from there to an offset blanket which, in turn, transfers the toner electrostatically to a sheet of paper or other printing medium. The third approach features the xerographic deposition of dry toner on the light-sensitive print member from which it is transferred directly onto the printing medium using a standard xerographic methodology.
There are several shortcomings associated with those prior art systems. They are designed primarily for short printing runs of simple subject matter. The quality of color image reproductions on these systems varies greatly in terms of chromaticity, resolution and density range. Also, prior art devices are typically quite limited in terms of speed of operation. More particularly, they are hindered by relatively long recording, writing and printing speeds. Further, although their set-up times are shorter than those of classical graphic art systems, their cost per page factors are significantly higher.
Furthermore, charged toner systems typically require toner particles with a relatively large toner size, i.e. greater than or equal to 5 micrometers, so that a uniform charge can be carried by the toner particles. Without the uniform charge, the toner particles become difficult to control and dusting problems arise.
We are also aware of printing apparatus which employs a print cylinder which functions both as an electrode and as a dielectric signal storage member. The print cylinder has a heated, dielectric, mildly ink phobic recording surface in rolling contact with a paper cylinder able to support a printing medium such as paper. Underlying that dielectric surface is a conductive layer which functions as an electrode when an image is being written or recorded on the print cylinder. Disposed around the print cylinder is a write station containing a print head, an inking station capable of dispensing different color thermoplastic inks and an ink transfer station which is actually the nip of the two cylinders. At the write station, a print head, responding to incoming data, deposits on the print cylinder during successive revolutions thereof, electronic latent images representing the color components or signatures of an original image, each such image being in the form of a pattern of electrostatic charge domains or spots whose field strengths vary in accordance with the gray scale or color values of the original image. As the print cylinder rotates, this charge pattern is advanced to the inking station where a heated inking head presents to the plate cylinder surface during successive revolutions of the cylinder, special thermoplastic inks whose colors usually, but not necessarily, correspond to the colors of the images being recorded on that surface by the print head. Usually for subtractive color printing, these colors include cyan, magenta, yellow and black.
When a recorded area on the print cylinder surface sweeps past the inking station, the field lines from the electrostatic charge domains or image spots comprising the latent image thereon take bites of molten ink from the inking head. The field lines may or may not momentarily change during passage under the ink head, depending on the presence of grounded or biased members of the ink head. The ink bite quantities are directly proportional to the field intensities of the charge domains. Thus, the print cylinder surface, despite its inkphobic nature, acquires variable quantities of ink at these image spots which are related to the field strengths at those spots thereby, in effect, developing the latent image on that surface. The ink is held by electrostatic forces to that surface as the developed images advance to the ink transfer station.
At the ink transfer station, the ink, still molten on the print cylinder, and the relatively cool paper on the paper cylinder are rotated through the nip of the two cylinders. At that line of contact, there is a phase transformation of the ink which causes the ink to switch from a liquid condition to a solid condition which results in the instantaneous transfer of the ink to the paper. This adherence and the ink-phobic nature of the cylinder surface overcome the electrical forces holding the ink to the plate cylinder so that there is substantially total transfer of the ink where the ink contacts the paper. As a consequence, the image printed on the paper supported by the paper cylinder corresponds exactly to the latent image impressed on the plate cylinder.
A printing apparatus of the above type is disclosed, for example, in U.S. Pat. No. 5,325,120, the contents of which is hereby incorporated by reference herein.
Very recently there has been developed by Dr. Manfred R. Kuehnle at XMX Corporation, Billerica, Mass. an entirely new printing technique which relies on dielectrophoresis. In accordance with this technique, electrostatic images may be recorded on a print cylinder or other print member using a print head similar to the one described in the above patent. In this case, however, the print member has an anisotropic recording surface so that the electrostatic charge domains applied to that surface by the print head produce non-uniform or nonhomogeneous electrostatic fields at each pixel position which fields extends above the surface of the print member. When those charged areas of the print member are moved opposite the developing medium, i.e., dielectric ink or toner, the field induces an electric dipole moment in that medium through dielectric polarization. The resulting polarized medium is pulled by the field gradient toward the region of highest field. In other words, the polarization charge at one end of the medium in the stronger field is pulled more strongly in the direction of the stronger field, while the opposite and equal polarization charge at the other end of the medium is repelled in the other direction more weakly because of the weaker field there. Thus, the developing medium travels to and adheres to those areas of the print member where the fields are strongest.
Dielectrophoretic printing thus provides electrostatic printing without having to use charged ink or toner particles. That is, while the developing medium is polarized in that the positive and negative charges on the medium are localized because of the presence of a non-uniform electrostatic field, the net charge on the medium is zero. Such uncharged medium, in contrast to the usual charged ink or toner particles, is not bound to the surface by image charge attraction or by interactions with a charge-induced polarization of the dielectric print cylinder. Therefore, it is easier to obtain a clean, fog-free developed image on the print cylinder as compared with the images developed by electrically charged inks or toner particles.
There are various ways of providing a non-uniform electric field on the dielectric surface of a print member such as a print cylinder. For example, as contemplated by Dr. Kuehnle, supra, one may write on the surface using a wire carrying a periodically varying voltage, e.g., AC or rectified AC, with the amplitude of the voltage varying in accordance with the digital input to the printing apparatus.
Alternatively, the non-uniform field applied to the print member may be due to the structure of the print member itself. More particularly, the print member can be provided with a dielectric surface which is anisotropic in that it has a pattern of conductive paths extending from the surface of the dielectric layer to a ground plane underneath that layer. One way of providing these grounded areas or field termination points on the dielectric layer is by forming that layer so that there is a multiplicity of crystallites which have so-called grain boundaries whose electrical conductivity is substantially higher than that within the crystallites themselves. These interface zones between the crystallites provide a periodic pattern of low-resistance paths through the dielectric layer to the ground plane thereby making the dielectric layer anisotropic. Resultantly, when electric charges are applied to the surface, say, by the microtunnel-type write head described in the above patent, the charges will arrange themselves on the surface of the print member to provide a maximum field strength surrounding each grounding point with a rapid fall off of the field strength between the ground points.
It would be desirable, however, to provide a print member such as this whose anisotropic characteristic does not depend upon the morphology or molecular structure of the dielectric layer.
In fields other than direct printing, dielectric surfaces have been placed on a metal roller to facilitate the transfer of uniform amounts of a charged toner. For instance, U.S. Pat. No. 5,315,061 describes a donor or developing roller for transferring a charged toner to a photoconductive belt to develop a latent image carried on the photoconductive belt. The donor roller is made of metal and small dielectric bodies are distributed on its surface. When a frictional charge is generated on the entire surface of the donor roller, electrostatic fields form between the dielectric bodies and the metal surface. Thus small closed electric fields--so-called "microfields"--are produced on the surface of the donor roller. These microfields facilitate the attraction of the charged toner to the donor roller surface. A doctor blade then regulates the toner to a uniform thickness.
The donor roller of U.S. Pat. No. 5,315,061 delivers a homogeneous and even amount of charged toner to permit development of an image on a photoconductive belt. No images are written directly on the donor roller, rather the images are written on the photoconductive belt.
U.S. Pat. No. 3,739,748 also shows a donor roller for transferring charged toner to a xerographic drum. The donor roller has a dielectric surface contacted by styli connected to a voltage source. The styli cannot write images on the donor roller, but rather can merely facilitate the gray scale rendition of the image which is written onto the xerographic drum by an exposing apparatus.
Neither of these donor rollers or their related apparati cause non-homogeneous microfields to exist above the surface of a print member.